Flat Tires

My friend at work just told me that my tires are really flat. I didn't even know I had to put air into the tires every 3-4 weeks! @_@ At first I thought I had a hole in my tires or something, but she said that it happens with cars because the air leaks out. We went downstairs just now to inspect the car and yeah, it looked really flat... I'm such a newbie!